we caught up with them at boston's symphony hall, part of the city's celebrity series, at the beginning of just their second-ever visit to the u.s. backstage, musicians aged 15 to 36 warmed up and chatted in a variety of languages. >> no, no, no! >> brown: at rehearsal, barenboim was a tough taskmaster... >> you see how much more space you have for a crescendo? please don't play mechanically. >> brown: ...cajoling his young musicians. >> you're just playing comfortably without any idea of thought. ya-ba-ba-ba. what is that? >> brown: i watched the rehearsal, and you were pretty tough on them at various points. you kept saying, "you're playing too comfortably." what does that mean? >> it means that to make music, to express music, you cannot adopt the line of least resistance. you have to adopt the line of most resistance. music is not politically correct. music demands total concentration and the perfect, perfect matrimony between thought, feeling, and gut. >> brown: so how? >> and people who think it's easy should choose another profession. >> brown: uncompromising in his music, barenb